The present invention relates generally to dewatering aqueous-solids by-product streams in alcohol production from grains and plant fibers, which by-product streams are commonly called stillage. More specifically, this invention relates to an improved method for dewatering such stillage to produce solids useful for animal feeds.
In the production of fuel alcohol from plant materials, the biomass is mixed with hot water to produce a wort, which is fermented until the final alcohol level is reached. The fermented contents are then typically discharged as a slurry to the beer well and from there to the beer still where the alcohol is removed by distillation. The remainder, after distillation, is known as the still bottoms or stillage, and consists of a large amount of water together with the spent solids.
Stillage in general has a complex composition; this is true especially for corn stillage, which includes the non-fermented fibers from the hull and tipcap of the corn kernel, as well as, particles of the corn germ with high oil content, oil and other lipids, the non-fermented portions of the corn kernel such as gluten, any residual unreacted starch, solubles such as proteins and enzymes, and the byproducts and residue of fermentation including dead yeast cells. The particle sizes range widely from broken parts of kernels 1-2 millimeters in size, down to fines in the under 10 micron range. Typically, stillage is dewatered to produce animal feeds. This feed-production process has added benefit of reducing waste disposal costs from the alcohol production.
FIG. 1 illustrates a conventional process for handling stillage, currently used in typical dry mill ethanol plants. Aqueous solids, such as whole stillage from corn, flow from the distillation column (which is not shown) along path 10 to a solid bowl decanter centrifuge 120 which separates the feed stream according to density into cake (the "heavier" substances) 121, and thin stillage (the lighter substances) 122. Since most corn solids are heavier than water, the cake 121 contains most of the solids. The thin stillage 122 typically has 5-6% solids of which about half are suspended solids, the remainder being dissolved solids including proteins, acids, unreacted sugars, and others. The suspended solids in the thin stillage are predominately fines but there is not a sharp cut-off since some larger particles are subject to carry-over with the liquid. Thin stillage is typically accumulated in a holding tank 129, from which typically 30-60% is recirculated as "backset" 130 to the cooking and fermentation stages to provide nutrients and to reduce the fresh water requirements. The remainder of the thin stillage 131 is sent to the evaporator 124 which concentrates the solids to a syrup 125 of typically 30-50% solids. This syrup is added to the cake and the combined stream 126 is, typically, sent to the dryer (not shown) to be dried to about 10-11% moisture.
The dewatering machinery which are generally most effective at producing high dry solids content, such as screen centrifuges and screw presses, have not proven feasible with corn stillage. Indeed, corn stillage and stillage from other grain fermentation has proven to be too fine and sticky for most separation devices. The typical industry practice has been to dewater such stillage using said solid bowl decanter centrifuges which are very functional, but which typically only produce cake solids content in the 30-35% range, in addition to having high electricity usage and high maintenance costs.
Numerous methods of overcoming this situation have been reported, such as separating most of the solids from the beer liquid prior to distillation so as to permit use of a screw press as described by B. J. Low in "The Efficient Production of High Quality Distiller's Dark Grains Using Stord Dehydration Process Technology. The separation step is followed by dewatering in a screw press to a solids content 50-54%, and then by drying in a special dryer. However, the presence of the alcohol at the separation step greatly complicates the drying process, requiring special closed-cycle dryers which are costly to purchase and expensive to maintain, as well as necessitating an alcohol vapor recovery system.
In some such ethanol production processes, such as in the production of ethanol from citrus residue as described in U.S. Pat. No. 4,952,504 issued to Pavilon, highly effective dewatering machinery such as screen centrifuges and screw presses (yielding dry-solids content typically 35-50% or higher) can be used to efficiently dewater solids filtered from the wort prior to fermentation. In fermentation from grains such as corn, however, this dewatering from the wort stage has the disadvantage of reducing the final alcohol yield.
U.S. Pat. No. 4,552,775, issued to Baeling, discloses a method for dewatering the stillage from a unique fermentation process which produces stillage of 20-30% dry substance (compared to the conventional corn fermentation which produces a stillage in the 5-12% solids range). This high solids stillage is combined with sufficient recycled dry product to obtain a 50-70% dry substance content which is then pelletized before drying in a through air dryer of special design. This method has the disadvantage that when applied to conventional stillages of 5-12% solids, the required recycle rate becomes very large, increasing the size and expense of the dryer.
A significant need remains for an improved, efficient and cost-effective method and apparatus to dewater conventional grain stillage, and in particular corn stillage, for the fuel alcohol industry.